1. Field of the Invention
The present invention relates generally to systems and methods for sound transduction. In particular, the present invention relates to the use of light signals for producing vibrational energy in a transduction pathway from a subject's tympanic membrane to the subject's cochlea.
A wide variety of hearing aids and ear pieces have been produced over the years to provide sound directly into a subject's ear. Most such hearing systems rely on acoustic transducers that produce amplified sound waves which impart vibrations directly to the tympanic membrane or ear drum of the subject. Hearing aids generally have a microphone component which converts ambient sounds into electrical signals which are then amplified into the sound waves. Telephone and other ear pieces, in contrast, convert and amplify electronic or digital signals from electronic sources into the desired sound waves.
Such conventional hearing aids and ear pieces suffer from a number of limitations. Some limitations are aesthetic, including the size and appearance of hearing aids which many users find unacceptable. Other problems are functional. For example, the production of amplified sound waves within the ear canal can result in feedback to the microphone in many hear aid designs. Such feedback limits the degree of amplification available. Most hearing aids and other types of ear pieces include an element large enough to obstruct the natural geometry of the ear canal, limiting the ability of natural sounds to reach the tympanic membrane and sometimes inhibiting the ear to respond to changes in ambient pressure. The precise shape of the external ear and the ear canal determine acoustic coupling of ambient sounds with the eardrum, determining in part the relative strength of various sound frequencies. An object inserted into the ear canal substantially changes this acoustic coupling, the person's perception of ambient sounds is distorted. These deficiencies can be a particular concern with the use of ear pieces in normal hearing individuals. Additionally, the acoustic coupling of the output transducers of many conventional hearing systems with the middle ear is often inadequate and seldom adequately controlled. Such deficiencies in coupling can introduce acoustic distortions and losses that lessen the perceived quality of the amplified sound signal.
An improved hearing system useful both as a hearing aid and an ear piece is described in U.S. Pat. No. 5,259,032. A magnetic transducer is held on a plastic or other support which is suspended directly on the outer surface of a subject's tympanic membrane by surface tension in a drop of mineral oil. The magnet is driven by a driver transducer assembly which receives ambient sound or an electronic sound signal and which generates an electromagnetic field, typically by passing electric current through a coil. The driver transducer will usually be disposed within the subject's ear canal, but could also be worn externally, as disclosed for example in U.S. Pat. No. 5,425,104.
The use of a magnetic transducer disposed directly on the tympanic membrane has a number of advantages. The risk of feedback is greatly reduced since there is no amplified sound signal. The coupling of the magnet or other transducer to the driver transducer is limited since the strength of the generated magnetic field decreases with distance rapidly, at a rate approximately proportional to the cube of the distance from the coil. The strength will conversely increase with the diameter of the coil. The inventions disclosed in U.S. Pat. Nos. 5,259,032 and 5,425,104 at least partly overcome these limitations. The two proposed designs attempt to provide enough electromagnetic coupling between the coil and the magnet to produce vibrations that are perceived as being sufficiently loud. As described in U.S. Pat. No. 5,425,104, a large coil around the subject's neck is used to drive the transducer and the ear canal is free from the presence of driving coil. The amount of current required to overcome the distance between the coil and the magnet in the eardrum has limited the usefulness of that approach. In the case of the small coil in the ear canal, the electromagnetic driving assembly must be very close to the eardrum (and yet not risk touching it) but the coil and its ferromagnetic core must be of such a size to effectively couple with the magnet that the driving assembly will affect the acoustics of the ear canal. Thus, while the magnetic transducer can be small enough to fit inside the ear canal, it will affect the natural sound shaping characteristics of the unobstructed ear.
Another limitation on the strength of the magnetic field produced by the coil is the need to align the axis of the driver coil and with the center of the coil and the center of the magnet on the eardrum transducer. The magnetic coupling will necessarily vary significantly with variations of such angle.
As a consequence the distance and the angle of the driver coil with respect to the magnet must be carefully controlled to avoid significant variations in magnetic coupling that would otherwise changes the perceived loudness produced with given amplitude of signal driving the coil. A further issue arises from the fact that the shape of the ear canal and the angle of the ear canal with the eardrum varies from person to person. Thus, in order to maintain a constant and precise coupling each and every time the subject inserts the coil assembly into the ear canal, it is necessary to consider embedding the coil driver assembly into a custom fitted mold which will position the coil assembly each time in the same relative position. Such custom assembly increases the cost of the products, and even relatively small pressure on the walls of the ear canal, which are very sensitive, can be uncomfortable (either during the insertion of the mold or while wearing it for extended period of time).
Various implantable hearing aids have also been developed which are unobtrusive and which generally avoid problems associated with feedback. For example, U.S. Pat. Nos. 6,629,922 and 6,084,957 disclose flextensional actuators which are surgically implanted to drive the ossicular chain (comprising the middle-ear bones) or the inner-ear fluid in the cochlea. U.S. Pat. No. 5,554,096 describes a floating mass transducer which can be attached to drive the mastoid bone or other element in the ossicular chain. Additionally, U.S. Pat. No. 5,772,575 describes the use of ceramic (PLZT) disks implanted in the ossicular chain of the middle ear. While effective, each of these devices requires surgical implantation and transcutaneous electrical connection to external driving circuitry. The internal electrical connection of the vibrating drive elements is potentially prone to failure over time and unless properly shielded, can be subject to electromagnetic interferences from common sources of electromagnetic field such as metal detectors, cellular telephone or MRI machines and the likes.
For these reasons, it would be desirable to provide hearing systems including both hearing aids and ear pieces which are unobtrusive, which do not occupy a significant portion of the ear canal from a cosmetic and an acoustical point of view, which provide efficient energy transfer and extended battery life, and which avoid feedback problems associated with amplified sound systems which are disposed in the ear canal. It would be further desirable if such hearing systems in at least some embodiments would avoid the need for surgical implantation, avoid the need for transcutaneous connection, provide for failure-free connections between the driving electronics and the driving transducer, and be useful in systems for both hearing impaired and normal hearing persons.
Finally, it would be useful if the amount of custom manufacturing required to achieve an acceptable performance could be minimized. At least some of these objectives will be met by the inventions described hereinbelow.
2. Description of the Background Art
Hearing transduction systems are described in U.S. Pat. Nos. 5,259,032; 5,425,104; 5,554,096; 5,772,575; 6,084,975; and 6,629,922. Opto-accoustic and photomechanical systems for converting light signals to sound are described in U.S. Pat. Nos. 4,002,897; 4,252,440; 4,334,321; 4,641,377; and 4,766,607. Photomechanical actuators comprising PLZT are described in U.S. Pat. Nos. 4,524,294 and 5,774,259. A thermometer employing a fiberoptic assembly disposed in the ear canal is described in U.S. Pat. No. 5,167,235. The full disclosures of each of these prior U.S. patents are incorporated herein by reference.
Materials which deform in response to exposure to light are known. The use of a photostrictive material (PLZT) to produce sound in a “photophone” has been suggested. The use of PLZT materials as light-responsive actuators is described in Thakoor et al. (1998), SPIE 3328:376-391; Shih and Tzou (2002) Proc. IMECE pp. 1-10; and Poosanaas et al. (1998) J. App. Phys. 84:1508-1512. Photochromic and other polymers which deform in response to light are described in Athanossiou et al. (2003) Rev. Adv. Mater. Sci 5:245-251; Yu et al. (2003) Nature 425:145; and Camacho-Lopez et al. (2003) Electronic Liquid Crystal Communications. Silicon nanomechanical resonant structures which deform in response to light are described in Sekaric et al. (2002) App. Phys. Lett. 80:3617-3619. The use of chalcogenide glasses which reversibly respond to light and can be used to design light-driven actuators is described in M. Stuchlik et al (2004). The full disclosures of each of these publications are incorporated herein by reference. The use of chalcogenide glasses as light-driven actuators is described in Stuchlik et al (2004) IEEE Proc.-Sci. Meas. Technol. 15: 131-136.